The present invention relates to an objective lens for an optical pick-up of an optical disc drive, which is capable of recording/reproducing data to/from a plurality of kinds of optical discs whose cover layers have different thicknesses.
In general, an optical pick-up for an optical disc drive is provided with a laser source such as a laser diode, an objective lens which converges the laser beam emitted by the laser source on a data recording layer of the optical disc to form a beam spot thereon, and a signal detecting system that receives the laser beam which is reflected from the data recording layer and passes through the objective lens. The signal detecting system produces various signals based on the received laser beam.
There are various types of optical discs on which digital information is recorded at various densities, respectively. For example, a DVD (digital versatile disc) has a recording density higher that that of a CD (compact disc) or a CD-R (CD Recordable) and has a cover layer thinner than that of the CD or CD-R.
When the recording/reproducing for the DVD having a higher recording density is performed, a smaller beam spot diameter is required on the data recording layer relative to the beam spot used for the CD having a lower recording density. For this reason, the optical pick-up is configured such that a NA (numerical aperture) is changed to a higher value to obtain a smaller beam spot diameter when the DVD is used and that the NA is changed to a lower value to obtain a larger beam spot diameter when the CD or CD-R is used.
In general, for the DVD, a laser beam having a wavelength range of 635–665 nm, which is lower than a wavelength range of 780–830 nm for the CD, is used because the beam spot diameter decreases as the wavelength of the laser beam decreases. When the CD-R is used, a laser beam having the wavelength of 780 nm is used due to reflectivity of a data recording layer of the CD-R.
As is well known, a condition of a spherical aberration in an optical system of the optical pick-up changes depending on a thickness of a cover layer of an optical disc being used. Therefore, it is required to correct the spherical aberration caused by replacement of optical discs having different thickness of cover layers.
Japanese Provisional Publication No. 2001-338431 discloses a CD/DVD compatible objective lens (i.e., an objective lens of an optical pick-up that can be used for both of the DVD and CD). The CD/DVD compatible objective lens has a diffracting structure on one of its lens surfaces. The diffracting structure has a plurality of concentric ring-shaped steps having a function of properly converging an incident beam on a data recording layer of an optical disc regardless of the thickness of a cover layer of the optical disc.
That is, the diffracting structure has wavelength dependence such that the spherical aberration changes depending on the wavelength of an incident beam. Consequently, the CD/DVD compatible objective lens is capable of supporting various types of optical discs having different thicknesses of the cover layers.
More specifically, a surface on which the diffracting structure is formed has an inner area which provides an NA required for an optical disc having relatively low recording density such as CD and has an outer area which is situated outside the inner area and which provides an NA required for an optical disc having relatively high recording density such as DVD.
The diffracting structure within the inner area has a function that a beam for CD is properly converged on the data recording layer of the CD and a beam for the DVD is properly converged on the data recording layer of the DVD. The diffracting structure within the outer area has a function that the beam for CD is not properly converged on the data recording layer of the CD and the beam for the DVD is most properly converged on the data recording layer of the DVD. Further, the diffracting structure within the outer area is configured such that a blazed wavelength is set substantially equal to the wavelength of the beam for DVD.
With the above mentioned structure, with regard to the beam for CD, only the beam for CD passed through the inner area properly converges on the data recording layer of the CD, while the beam for CD passed through the outer area diffuses on the data recording layer of the CD. Thus, the beam spot having relatively large diameter is formed on the data recording layer of the CD.
With regard to the beam for DVD, both of the inner and outer areas converge the beam passing therethrough on the data recording layer of the DVD. Thus, the beam spot having relatively small diameter is formed on the data recording layer of the DVD.
As described above, the CD/DVD compatible objective lens has excellent optical performance that it can be used for both of the DVD and CD and that the spherical aberration is corrected for both of the DVD and CD. However, the CD/DVD compatible objective has a problem that the beam for CD passed through the outer area of the objective lens may be detected by a detector of a servo mechanism after the diffused light is reflected from the data recording layer of the CD. If the diffused light reflected from the data recording layer of the CD is detected by the servo mechanism, a noise may be generated on a servo signal such as an AF (auto focus) signal, a TE (tracking error) signal and the like. In such a case, high-precision servo operation (i.e., optical pick-up operation) may be adversely affected.